Talk:Visby lenses

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What needs to be added[edit]

The lenses were dated to the 10th century, but when where they found, and who found them? Where can they be seen now? How do we 'know' what they were for? ~Bengaley

On a funny note--did anyone else get the mental image of the stereotypical hairy scary Viking with a monocle? It popped into MY head, anyway... 142.157.62.144 (talk) —Preceding undated comment was added at 22:01, 2 September 2008 (UTC)

A small dose of cold water ...[edit]

A lens with a focal length of 22 mm and F-number of 0.5 is practically useless for building a telescope, spherical aberration or not. It would do better as a burning glass, although for that purpose spherical aberration is irrelevant, diameter is everything (and 50 mm diameter is ok-ish, not great.)

The thing that these lenses would be good for, if indeed they are lenses, is as a "chart glass". That is, you lay it on a page of text and read a magnified view of the text. Good for reading fine print on nautical charts, and probably ok for reading books when your eyes are failing (but rather awkward compared to spectacles.)

On the other hand, so far as I am aware 12th century Vikings didn't have books ... -- 202.63.39.58 (talk) 17:33, 16 September 2009 (UTC)

Please correct the units[edit]

The second paragraph under "Description" mentions "an angular resolution of 25–30 μm", which is as self-evidently ridiculous as anything ever gets. Angular resolution is measured in angular units and not linear ones (duh). Maybe what's quoted is the spatial equivalent of angular resolution, but if so, then please also indicate where it is measured with respect to the lens. — Preceding unsigned comment added by 78.83.51.215 (talk) 17:38, 4 October 2011 (UTC)

Moreover, there is no reference for this measurement. In the cited papers, I found only the numbers for the variation of focal length with aperture (1mm for the best specimen — one which has not been preserved, and is discussed only based on a photo). Additionally, in the references they discuss only the optical performance of lenses with similar and exactly elliptical shapes, not the shapes of the actually present specimens. The residual variation w.r.t. best-fitting ellipse is about 0.4mm–1.5mm. For comparison, recall that usually required precision of optically sound refractive surfaces is below wavelength/2, so below 0.0003mm. — iz